1. Field of the Invention
The present invention relates to a small channel heat exchanger, and more particularly, to a two-phase small channel heat exchange matrix that provides for simultaneous heat transfer and mass transfer at a single, predetermined location in a separation column, whereby the thermodynamic efficiency of the separation process is significantly improved.
2. Background of the Invention
Separation processes are an integral part of chemical manufacturing and petroleum refining operations. In these industries, a large percentage of the energy consumed and capital expended is associated with the critical steps of refining and product recovery. Many separation processes are known, including distillation, absorption, membrane separation, evaporation, liquid-liquid extraction, and crystallization. Distillation, which involves the separation of components on the basis of their relative volatility, is the product recovery and purification technology most often used.
Distillation involves the separation of the components of a feed solution by countercurrently contacting a rising vapor phase with a downwardly flowing liquid phase. During the countercurrent contact, the more volatile components of the feed solution are stripped from the liquid phase by the hot rising vapor, and the less volatile components of the mixture are stripped and condensed from the vapor phase by the cold descending liquid. In the majority of distillation systems, some of the condensed vapor is returned to the upper rectifying section of the column for a continuous downward liquid flow. The condensed vapor returned to the column is referred to as liquid reflux. The feed solution is generally introduced into the column between the upper rectifying section and the lower stripping section.
A conventional distillation apparatus includes at least one column containing vapor-liquid contacting elements, such as packing or trays, that provide the surface areas within the column for the mass transfer between the vapor and liquid phases of the feed solution. Packing can be structured or random, including a structured honeycomb configuration of sheet metal or single honeycomb cells, glass beads, or ceramic rings. Trays are substantially flat plates placed horizontally at preselected heights within the column. Trays having caps, valves, and/or perforations can further be equipped with liquid distributors and/or liquid inlet/outlet ports, and are often used in conjunction with liquid downcomers that facilitate the flow of liquid from an upper to a lower tray.
The mass transfer between the vapor and liquid phases is facilitated by the vapor-liquid contacting elements, which provide surface areas for the interfacing of the liquid and vapor during the countercurrent flow. In a packing-type column, the liquid flows downwardly as a film over the surfaces of the packing, contacting the vapor as it rises through the voids in the packing. In a tray-type column, the liquid collects in shallow pools on the trays and contacts the vapor as it bubbles up through the perforations. Conventionally, the packing or trays are designed to provide a surface for mass transfer only, not heat transfer.
In distillation, the mass-separating agent is energy, in the form of heat. Heat transfer is typically achieved by operating an external reboiler at the bottom of the column and an external condenser at the top of the column. The energy balance within the column is dependent upon the energy introduced into the system by the reboiler and the energy removed from the system by the condenser. As the contact between the vapor and liquid phases within the column is essentially adiabatic, the reboiler and condenser provide the energy input and output, respectively, for the heat transfer process that dictates the mass transfer between the liquid and vapor phases.
The operating variables of a distillation column include the temperature and feed rate of the feed solution, and the amount of heat generated by the reboiler and removed by the condenser. Importantly, precise control of the operating variables is required to regulate the local working conditions of the column which are determinative of the equilibrium state and thermodynamic efficiency of the column as a whole. For example, the local equilibrium temperature at different points within the column determines the local saturation pressure, which influences the mass transfer between the vapor and liquid phases. Changing the local equilibrium temperature changes the amount of vapor and liquid available for condensation and evaporation, respectively. During normal operation, when the flow of liquid and vapor are not at the proper local flow rates, excessive flooding of liquid (weeping) occurs, which impairs the effectiveness of the vapor-liquid contacting elements and the efficiency of the mass transfer between the vapor and liquid phases. The improper balance of vapor flow and liquid flow over the length of the column causes the reboiler and condenser to work harder, requiring more energy to maintain the column in an equilibrium state. Thus, problems associated with distillation columns are the inability to precisely control the conditions within the column and the inherent energy inefficiency of the column design.
A common approach that addresses the need to control the local working conditions of the column includes distributing the heating and/or cooling along the length of the column rather than supplying all the heat in the reboiler at the bottom of the column and removing all the heat at the top in the overhead condenser. Current heat exchanger designs are too great a size to be located internally within the column, and are instead provided externally, including supporting structures. For example, heat exchangers can be positioned at intermediate points outside of the column, whereby liquid is drawn off at a certain column height, pumped to the heat exchanger, heated, and returned to the column. This solution, however, requires a more complex and expensive column design, increasing capital and maintenance costs. Also, energy is lost along the auxiliary path. Most importantly, the external heat exchanger design accommodates only heat transfer and not mass transfer between the liquid and vapor phases.
A need exists for an intermediate heat exchange mechanism that is an integral part of the distillation column, such that heat transfer and mass transfer are accomplished simultaneously and at the same location within the column.
The present invention, a two-phase small channel heat exchange matrix, involves a distillation column design incorporating small channel heat exchange technology that overcomes the energy inefficiency and equilibrium control problems experienced in the prior art. The matrix is also simple to construct and maintain, and adaptable to diverse separation applications. The matrix is comprised of a series of small tubes or channels for transporting a two-phase coolant. Each channel has an exterior surface, an interior surface through which the coolant flows, and a hydraulic diameter no greater than 5.0 mm. The channels are sufficiently spaced apart to allow vapor to ascend between adjacent channels within the matrix. In operation, liquid flows across and/or downwardly along the external channel surfaces, as vapor flows upwardly between the channels. The external surface areas of the channels are the liquid-vapor contacting elements upon which the mass transfer between the liquid and vapor phases occurs. At the same time, a two-phase coolant is circulated through the interior of the small channels to maintain a desired temperature and uniform heat transfer across the matrix.
The matrix is positioned inside the distillation column at a predetermined height, and the temperature of the coolant flowing through the channels of the matrix is monitored to control the local temperature and pressure within the column. In this way, mass transfer and heat transfer are achieved locally and simultaneously. Mass transfer is facilitated by providing vapor-liquid contacting surfaces in the form of the outside surfaces of the individual cooling channels comprising the matrix. Heat transfer is facilitated by introducing heat or cooling via the fluid flowing through the matrix channels, resulting in a much higher heat transfer surface-area-density ratio (.ltoreq.1000 m.sup.2 /m.sup.3) and more uniform heat distribution.
Importantly, the matrix minimizes the change in entropy of the liquid-vapor system within the column by controlling the change in local temperatures and saturation pressures, as well as the change in velocity of the liquid and vapor flow. Thus, partial condensation and vaporization occurs at a higher rate than in the conventional designs. The matrix minimizes entropy and achieves greater thermodynamic reversibility, resulting in a more energy efficient distillation process.
The matrix increases the energy efficiency of the column because heat and mass transfer are accomplished in one step, resulting in improved separation efficiency per unit power consumption. The matrix also promotes internal liquid reflux as a result of the ability to vary the local equilibrium along the height of the column. Thus, condensed vapor (liquid reflux) is continuously returned to the system internally. The more energy efficient design allows for a single column to be used, contributing to process plant intensification and a reduction in capital expenditures, as the need to route liquid and vapor to external columns, reboilers, condensers, and heat exchangers may be eliminated.
The matrix further allows precise control of the local equilibrium state, overcoming the inherent control problems associated with start-up and upset conditions experienced in conventional column designs. Importantly, temperature is controlled locally, at several points within the column, even in large scale separation processes. By regulating the pressure and the temperature of the coolant loop system, the velocity and the phase of the coolant flowing through the small channels can be controlled, and the local column equilibrium can be maintained at a desired value. Precise heat control of the local equilibrium state also enables more accurate management of the cut point temperature, resulting in a higher quality product. Furthermore, column failures caused by the improper balance of mass transfer within the column are prevented, or, alternatively, quickly detected and corrected. Such rapid response to pressure changes within the column and upset conditions precludes the need to wait for the entire column to establish equilibrium before modifications to the column can be made.
Finally, the invented two-phase small channel heat exchange matrix simplifies column design and allows for the universal application of the matrix in existing separation columns without restricting the use of the column to the particular separation for which the column was initially designed. In conventional column designs, the number and configuration of trays or packing, the reboiler capacity, incorporation of auxiliary heat exchangers, feed stream composition, and quantity and quality of the final product must all be carefully considered and are determinative of the cost of the separation process. The matrix, however, is a universal design which can be retrofit into existing separation columns at one or more predetermined column heights. By using the matrix, smaller columns may replace more complex systems, and reliance on overhead condensers and/or reboilers can be reduced. Control instrumentation can be simplified and more economically designed to accommodate local changes in column equilibrium. The matrix also allows for additional draw-off points of products at each particular cut-point.
Therefore, in view of the above, a basic object of the present invention is to provide to a two-phase small channel heat exchange matrix that improves the energy efficiency and allows precise control of the local equilibrium conditions of a separation process.
Another object of the invention is to provide a two-phase small channel heat exchange matrix that provides for heat transfer and mass transfer simultaneously and at a single location within a separation column.
Another object of the invention is to provide a two-phase small channel heat exchange matrix that minimizes the change in entropy within a separation column.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.